The invention relates to a chemical flooding process for producing oil from a subterranean reservoir. More particularly, it relates to such a process which is best suited for use in an oil reservoir in which the predominant aqueous liquid has a dissolved salt content of more than about 9%.
As indicated in a paper presented at a symposium on improved methods for oil recovery in April 1978, SPE Paper No. 7053, two essential criteria that must be met for successful recovery of residual oil by chemical flooding are (1) very low interfacial tension between the chemical bank and the residual oil and between the chemical bank and the drive fluid, and (2) small surfactant retention losses to the reservoir rock. The paper relates to the phase behavior of such a chemical bank (or microemulsion or aqueous surfactant system) as a function of salinity and describes how the salinity of the drive water or fluid (or mobility buffer fluid) tends to control the amount of the surfactant retention.
In a collection of papers, "Improved Oil Recovery By Surfactant Flooding," D. O. Shah and R. S. Schechter, Academic Press, New York 1977; a paper by S. P. Trushenski, "Micellar Flooding: Sulfonate-Polymer Interaction", considers the sulfonate-polymer incompatibility in a chemical flood process in which the reservoir oil and water are displaced by a micellar fluid followed by a polymer-thickened mobility buffer bank. That paper indicates that the interaction between the sulfonate and polymer tends to increase the sulfonate requirement and the extent of that interaction increases with increases in the drive water salinity, regarding drive waters thickened by either bipolymers, i.e., xanthan gum polymers, or partially hydrated polyacrylamide polymers.
Although, in some reservoirs, the extent of the polymer-surfactant interaction can be reduced by reducing the salinity of a polymer-thickened mobility buffer liquid relative to that of an aqueous surfactant system, in reservoirs which contain highly saline waters, such a reduction in salinity may not be feasible. For example, in U.S. Pat. No. 4,074,755 by H. J. Hill, J. Reisberg, F. G. Hellfrich, L. W. Lake and G. A. Pope, it is pointed out that, although numerous chemical flooding procedures have been suggested for utilizing relatively low salinity surfactant systems and/or polymer containing mobility buffers, such procedures have often been unsucessful. In such procedures the salinities of the injected fluid are often drastically altered by interactions within the reservoir. Among the more important physical and chemical mechanisms which operate within typical oil bearing reservoirs are the cross flow of fluids between layers of different permeability, the dispersive mixing between the fluids being displaced and the displacing fluid, the dissolving of minerals when a water differing from the formation water contacts the reservoir rock, and the cation-exchange reactions between the reservoir rocks and the injected water. The patent also mentions that when the reservoir water is highly saline, or contains large proportions of multivalent cations, it may be impossible, or at least uneconomical, to formulate an active surfactant system having an ionic composition (or salinity) equivalent to that of the reservoir water.
In an aqueous surfactant system the capability of forming an effectively low interfacial tension against the reservoir oil frequently occurs within a range of salinity such that the system forms three equilibrium phases when contacted by the oil and brine of the oil-containing reservoir. In such a situation a surfactant-rich phase, which may contain a large amount of both oil and brine, may be in equilibrium with essentially pure brine and essentially pure oil. The salinity at which the system is capable of forming the lowest interfacial tension is called the optimum salinity. For use in a reservoir in which the brine contains both divalent and monovalent cations, the optimum salinity for a surfactant system that contains a given surfactant material usually increases with increasing surfactant concentration. But, the increasing of the surfactant concentration increases the expense of the oil recovery operation. In addition, the extent to which the salinity of the surfactant system can be increased is limited by the concentration at which that system forms two phases (in the absence of the reservoir oil).
It is known that certain surfactants may exhibit an optimum or adequate oil-displacing activity in aqueous liquids which are highly saline. Surface active alkylaryloxypolyethoxyethane sulfonate surfactants exhibit such a capability and their use in oil recovery processes has been previously proposed. For example, U.S. Pat. Nos. 4,018,278 and 4,088,189 describe (a) using such sulfonates as the predominant surfactants for imparting an oil-displacing efficiency within a highly saline aqueous surfactant system, and (b) using them in the concentrations indicated by capillary displacement tests or interfacial tension measurement tests designed for determining the economically efficient proportions to be used in aqueous liquids which are the same as or have salinities equivalent to those in highly saline reservoirs. U.S. Pat. No. 4,066,124 describes uses of such sulfonates as co-surfactants in relatively saline aqueous surfactant systems containing mixtures of predominantly water-soluble and predominantly oil-soluble petroleum sulfonate surfactants. U.S. Pat. No. 3,977,471 describes such surfactants as members of a new and improved class of surface active agents for use in chemical flood processes in reservoirs containing water with salinities of 2% or more. U.S. Pat. No. 3,827,497 describes waterflood surfactant compositions that contain sulfonated oxyalkylated alcohols and indicates that an alkylaryloxypolyethoxyethane sulfonate can be included. U.S. Pat. No. 4,077,471 describes injecting mixtures of said sulfonates with oil soluble non-ionic surfactants in highly saline aqueous mixtures ahead of polymer-thickened aqueous mobility buffer or drive fluids "if no adverse interaction occurs between the polymer and the surfactant."
U.S. Pat. No. 3,653,440 by J. Reisberg describes a water-flood oil production process in which the oil is displaced by injecting an active surfactant system followed by drive liquid comprising a mixture of a gas and an aqueous liquid, with the gas and liquid being injected so that the drive fluid mobility is relatively low and the gas moves ahead of the liquid. The patent indicates the desirability of having a surfactant in such a liquid to provide a gas-water surface tension at least as low as about 30 dynes per centimeter, so that the gas bubbles are relatively small and homogeneously distributed throughout the liquid.